Projection exposure apparatuses for microlithography generally consist of a light source, an illumination system, which processes the light rays emitted by the light source, an object to be projected, generally called reticle or mask, a projection lens, called lens for short hereinafter, which images an object field onto an image field, and a further object, onto which projection is effected, generally called wafer. The reticle or at least one part of the reticle is situated in the object field and the wafer or at least one part of the wafer is situated in the image field. The lens generally defines an optical axis with respect to which the optical elements belonging to the lens are arranged. Generally, the optical elements are rotationally symmetrical with respect to the optical axis and the optical axis is a normal to the object field and image field. The design of the lens is called rotationally symmetrical in this case.
If the reticle is situated approximately completely in the region of the object field, and the wafer is exposed without a relative movement of wafer and image field, then the projection exposure apparatus is generally designated as a wafer stepper. If only a part of the reticle is situated in the region of the object field, and the wafer is exposed during a relative movement of wafer and image field, then the projection exposure apparatus is generally designated as a wafer scanner.
During the exposure of the wafer, the projection exposure apparatus is operated with a predefined aperture and a setting predefined by the illumination system, for example a completely coherent, partly coherent, specifically dipole or quadrupole setting. The aperture is predefined by the illumination system and/or defined by a stop in the lens. Customary image-side apertures for lenses for microlithography are values of between 0.5 and 0.6, or 0.6 and 0.7, or 0.7 and 0.8, or 0.8 and 0.9, or else higher. The setting is generally predefined by optical elements of the illumination system such as, for example, an axicon, a stop or a micromirror array or one or more changeable DOEs (diffractive optical elements). During exposure, from each field point associated with the object field, a maximum light beam trimmed by the aperture stop passes from the object field to the image field. In an ideally manufactured lens, the imaging aberrations of which are determined only by the design of the lens, the wavefront defined by the maximum light beam in the vicinity of the image point associated with the field point approximately corresponds to a spherical wave with the image point as central point. The possible resolution of such a lens is therefore determined by the diffraction orders which still lie within the aperture. Therefore, such lenses are also called diffraction-limited.
If the region between the last optical element of the lens and the wafer is filled with a gas as medium, then the refractive index thereof is generally approximately 1.00 and the above apertures are therefore both geometrical and numerical.
If the region between the last optical element of the lens and the wafer is filled with a liquid as medium, then this is referred to as an immersion lens. One possible immersion liquid is water, which has a refractive index of approximately 1.43. Therefore, the image-side apertures indicated above have to be increased by the factor 1.43 in order to determine the assigned image-side numerical apertures. This therefore results in image-side numerical apertures for immersion lenses of approximately 0.75 to 0.9 or 0.9 to 1.05 or 1.05 to 1.2 or 1.2 to 1.35 or else higher.
The possible resolution R that can be achieved with such a lens for micro lithography is inversely proportional to the numerical aperture NA and proportional to the operating wavelength λ of the lens and a process parameter k1:
      R    =                  k        1            ⁢              λ        NA              ,where k1 is always at least 0.25. The operating wavelength is generally 365 nm, 248 nm, 193 nm or 13 nm. In the case of 13 nm, the lenses are purely catoptric lenses, that is to say lenses consisting only of mirrors. These are operated in a vacuum with geometrical—and correspondingly numerical—apertures of 0.2 to 0.25 or 0.25 to 0.3 or 0.3 to 0.4 or 0.4 to 0.45 or higher.
Further types of lenses for microlithography are dioptric lenses, that is to say lenses consisting only of lens elements, and catadioptric lenses, that is to say lenses consisting of lens elements and mirrors.
During the operation of the projection exposure apparatus with light having the operating wavelength, changes arise in the optical elements belonging to the lens of the projection exposure apparatus, which lead to, in some instances irreversible, changes in the optical properties of the lens. By way of example, mention shall be made here of compaction, rarefaction and chemically governed changes of possible coatings of the optical elements. Further, irreversible changes are produced by drifts of optical elements in the mounts thereof, the drifts being established with increasing time. Other changes are of a reversible nature such as e.g. lens element heating with the thus implied change in shape and the change in the distribution of the refractive index of the lens element. These lead to time- and location-dependent changes in the optical properties of the lens.
Therefore, lenses for microlithography have been supplemented with an increasing number of manipulation possibilities in the course of their development. These possibilities can be used to counteract the changes in the optical properties of the lens in a controlled manner. Use is made of manipulators which displace, rotate, exchange, deform, heat or cool one or a plurality of optical elements associated with the lens, such as lens elements, mirrors or diffractive optical elements. In particular, aspherized plane plates are provided as exchange elements in the lens. Exchange elements can also be optical elements of a lens which are provided with manipulators. These elements are preferably some of the first and last optical elements of the lens as seen in the direction of light propagation, or some of the optical elements situated in the vicinity of an intermediate image of the lens, or some of the optical elements situated in the vicinity of a pupil plane of the lens. The term vicinity is defined here with the aid of the so-called subaperture ratio. In this respect, cf. WO2008034636A2, for example, which is hereby incorporated within its full scope in this application. In particular pages 41 and 42 therein shall be incorporated within their full scope in this application.
Thus, by way of example, WO2008037496A2 discloses a lens for microlithography containing an optical element to which a multiplicity of forces and/or moments are applied by a manipulator, such that the optical element attains a high local variability with regard to its form.
Manipulators which deform an optical element are distinguished by their particularly fast response behavior. R. K. Tyson: Principles of Adaptive Optics, Academic Press, Inc., ISBN 0.12.705900-8, gives a general introduction to rapidly responding manipulators from the field of telescope technology.
Thus, by way of example, WO2008034636A2 discloses a plane plate in a lens for microlithography. Conductor tracks to which current can be applied are situated in or on the plane plate. In the case of the change in temperature caused thereby, the refractive index of the plane plate can be influenced locally, such that the plane plate has a high local variability with regard to its refractive index.
Thus, by way of example, in WO2009026970A1 the plane plate from WO2008034636A2 is provided with a thermal sink that makes possible a temporal constancy of the spatially averaged temperature of the plate.
Thus, by way of example, EP851305B1 discloses a pair of plane plates, so-called Alvarez plates, in a lens for microlithography. This pair of Alvarez plates has an asphere in each case on the mutually facing surfaces of the plates, the aspheres compensating for one another in terms of their optical effect in a relative zero position of the plates with respect to one another. If one or both of the plates is or are deflected perpendicularly to the optical axis of the lens, then the effect of these Alvarez plates is established.
Thus, by way of example, EP1670041A1 discloses a device which serves for the compensation of image aberrations that are introduced into the lens for microlithography specifically as a result of the absorption of dipole illumination. An optical element situated in a pupil plane of the lens experiences non-rotationally symmetrical heating in the case of dipole illumination. The optical element is subjected to additional light from a second light source, which emits light preferably having a different wavelength from that of the operating wavelength, at least approximately complementarily to the heating. Undesired image aberrations are thereby compensated for, or at least reduced, or converted into other image aberrations, which are qualitatively different from the former. In this case, a first image aberration should be understood as qualitatively different from a second image aberration if the indices of the coefficients—which differ significantly from zero—of the expansions of the image aberrations into Zernike polynomials differ in pairs. For the expansion of an image aberration into Zernike polynomials, cf. DE102008042356A1 and DE102004035595A1.
Thus, by way of example, in DE19827602A1 an optical element is subjected to cold or heat over its circumference via Peltier elements. In this case, specifically in the case of manipulators which apply heat to an optical element, the following effect can be observed: these manipulators are used for correcting image aberrations which arise as a result of the fact that a plurality of optical elements of the lens become heated. In general, in this case an individual optical element to which heat is to be applied is intended to compensate for a plurality of such optical elements that become heated. This has two consequences:
1. Firstly, it is necessary for a relatively high heat to be applied to the optical element in comparison with each individual one of the optical elements to be compensated for. Therefore, the manipulator effect of the optical element to which heat is to be applied can no longer be assumed to be proportional to the deflection thereof.
The term “linear” is used instead of “proportional” below.
2. Secondly, a hysteresis effect is manifested during the compensation: if heat is applied to the optical element at a first position, whereby the heating of a first optical element is intended to be compensated for, and if the optical element is heated at a second location, different from the first location, whereby the heating of a second optical element is intended to be compensated for, then the heat distribution that arises after these two heatings in the optical element, and thus the manipulator effect, is dependent on the temporal order in which these two locations are heated.
These two effects are not limited to transmissive optical elements, such as lens elements, for example. In the case of mirrors, too, which are used in particular in EUV lenses and have a main body composed of Zerodur or ULE, non-linearity and hysteresis can be observed. Non-linearity and hysteresis arise in the case of the mirrors by virtue of the fact that the magnitude of the surface deformation caused by the heating is directly dependent on the heating, on the one hand, but also influences the gradient thereof, on the other hand, since the coefficient of thermal expansion of a mirror material such as Zerodur or ULE itself is again temperature-dependent.
These two problems, non-linearity and hysteresis, can be combated as follows:
The thermal manipulator from WO2009026970A1 has the advantage over the thermal manipulators from WO2008034636A2, EP1670041A1, and DE19827602A1 of a temporally compensated heat balance caused by its thermal sink. This has the consequence that the thermal manipulator from WO2009026970A1 for small deflections around its temporally and spatially averaged temperature can be assumed to be linear in terms of its optical effects since the latter always vary in a predefined temperature interval that does not change during the history of the manipulator.
In this case, the optical effect of a manipulator, for a predefined deflection of the manipulator, should be understood to mean the difference in the image aberrations of the lens between deflected and non-deflected manipulator. If a standard deflection that is relatively small in comparison with the maximum possible deflection range of the manipulator is predefined, then the optical effect is also designated as the sensitivity of the manipulator. In this case, the deflection of the manipulator is understood to be a vector whose dimension corresponds to the number of degrees of freedom of the manipulator and whose entries describe the intensity of the deflections in the individual dimensions.
By way of example, in EP1670041A1 8 infrared heat sources are directed at a lens element. The deflection can therefore be described as an 8-dimensional vector having entries of heat flows to be set in joules/second, multiplied by the duration of the heat flows of the respective sources in seconds. The image aberrations arising as a result of these heat inputs can be measured or simulated and related to a lens to which heat is not applied. This results in the optical effect of the manipulator.
A further advantage of the thermal manipulator from WO2009026970A1 over the thermal manipulators from WO2008034636A2, EP1670041A1, and DE19827602A1 is its freedom from hysteresis or, to put it another way, its property “to forget”. This should be understood to mean the following: the thermal manipulator from WO2009026970A1, in the case of a deflection to be performed, yields an optical effect which is independent of its present deflection state since, in order to attain this deflection state, the total heat introduced into the plane plate by the manipulator has already flowed away again via the thermal sink. Only the spatial relative temperature distribution in the plane plate, which distribution is relevant to the present optical effect, yields the initial temperature distribution for a renewed deflection of the manipulator and the associated renewed redistribution of the temperatures in the plane plate.
By contrast, the thermal manipulators from WO2008034636A2, EP1670041A1, and DE19827602A1 have to be referred to as non-linear and non-forgetting manipulators. The optical element to which heat is applied by the manipulator at a first instant emits heat to its surroundings in an undefined manner. This has the consequence that at a second instant of subsequent application of heat, it is unclear what temperature distribution is currently present in the optical element. On account of the non-linearity of the change in the surface of the optical element, such as, for example, a mirror in the case of EUV, with a use of Zerodur or ULE as mirror material, depending on the temperature, this means that, on the one hand, the optical effect of this new heat input is no longer linearly dependent on the intensity of the deflection; on the other hand, in mirrors and also in other optical elements such as lens elements, for example, the optical effect thereby also becomes dependent on the initial temperature distribution in the optical element, and thus in particular on the history of the manipulator.
It should be emphasized that the non-thermal manipulators mentioned above likewise forget their history.